Length is a convenience class used for a couple of things. It can be used to specifiy unit type, and can be used for conversion from radians to/from whatever units are represented by the implemented class.

Class for calculating overlap score using the length of the word. The calculation of the overlap score will be equals to the sum of the lengths of all the word occurrences producing an overlap. author Francisco Viveros-Jiménez

This is a bit annoying to add this to the set of expressions, but we need it to translate for expressions, because the length primitive is generic, but the for loop always uses the actual length of the vector. (is this another sign we need to push 'fors' down into the IR level?)